Is moss-associated nitrogen fixation controlled by the same factors across shoots, species and sites?

Biological N₂ fixation performed by moss-associated cyanobacteria is among the main sources of new nitrogen (N) input in pristine subarctic ecosystems. Yet, until now there has been a lack of knowledge on the drivers of biological N₂ fixation on small spatial scales (moss segment) in relation to the drivers at larger scales (moisture ecosystem). To investigate this, we assessed the capacity of N₂ fixation along shoots of two common moss species, Pleurozium schreberi and Hylocomium splendens, collected along a steep precipitation gradient in the Subarctic. Furthermore, concentrations of nutrients and pH were analysed along the same moss shoot-gradients. We found the highest nitrogenase activity in H. splendens collected at the location with the highest mean annual precipitation. Nitrogenase activity varied significantly along the moss shoots, and between species and location along the precipitation gradient. P. schreberi had the highest nitrogenase activity in the middle segments, while for H. splendens, it was highest in the lowest segments (below 3 cm). Contents of iron, molybdenum and N generally increased with moss segment depth, but phosphorus concentrations decreased and pH was stable across segments. Taken together, the factors that drive nitrogenase activity at small scales differ between moss species, whereas precipitation predominantly controls nitrogenase activity at larger scales (across habitats).